Document EP-0475270 discloses a puffer-type circuit-breaker including a casing filled with a dielectric gas under pressure, and two arcing contacts that co-operate with each other, at least one of which is part of a moving contact assembly that is secured to a drive member, and that is capable of being displaced axially inside the casing between a closed position and an open position, the moving contact assembly being constituted by a first tube and a second tube that is coaxial with the first tube so as to define a blast chamber on one side of a first annular wall interconnecting the first tube and the second tube, and a compression chamber on the other side of said annular wall, which compression chamber communicates with the blast chamber and is closed by a semi-moving piston.
During opening, the gas in the compression chamber is compressed because the distance between the piston and the annular wall separating the compression chamber from the blast chamber decreases. In that known circuit-breaker, the dielectric gas is compressed during the entire displacement of the moving contact assembly from the closed position to the open position. A certain quantity of energy is required to displace the moving contact assembly. The required quantity of energy is particularly large since the gas in the compression chamber is compressed during the entire travel of the moving contact assembly.